Transistor Made to Run on Protons

Squid shell is the secret ingredient in a device that modulates a proton current

21 September 2011—It turns out the transistor has a positive side. A team at the University of Washington, in Seattle, has created the first solid-state transistor that controls the flow of protons instead of electrons. The device could help pave the way for gadgets that can interface at a molecular level with living systems, since biology commonly employs protons and ions to perform work and transmit information.

Unlike the lightweight electron, which is easily knocked off atoms and can flow freely through a wide variety of metals and semiconductors, the proton has proved far more difficult to harness. Researchers interested in creating a device that can modulate the flow of protons or heavier ions typically rely on water, creating microfluidic channels where they can manipulate dissolved ions.

But there’s no reason why a proton-powered solid-state field-effect transistor (FET)—one that can modulate the flow of current—can’t be built, the UW researchers reasoned. The trick is to find the right materials to conduct protons in the channel and in the contacts that form the device’s source and drain.

Postdoc Chao Zhong and graduate student Yingxin Deng have created the first device that seems to do the job. They built a transistor with a 3.5-micrometer-wide channel made from nanofibers of chitosan, a compound that was originally derived from an internal, shell-like structure in squid. Chitosan does not transmit protons on its own, but when it’s loaded with proton-donating acid groups and exposed to fairly humid air, it creates a water-saturated fiber full of free protons that can move when exposed to an electric field.

"As far as we know, it’s the first FET that directly controls and measures the proton current," says team leader Marco Rolandi. The results were published on Tuesday in Nature Communications. Rolandi says he hopes that similar proton transistors can be used for direct sensing of cells in the laboratory and, ultimately, to make devices that can stimulate ion channels and other signaling pathways inside organisms. To be biocompatible, the transistor, which is built on silicon, would have to have a different substrate. Researchers would also have to develop a compact way of delivering proton-donating hydrogen to the contacts, Rolandi says.

In many ways, the proton transistor looks like a traditional FET; in it, current flows between the source and drain, through a channel, all under the control of the gate. Applying a negative voltage to the gate created as much as a fivefold increase in the concentration of protons in the channel, and by extension, the conductivity of the device. Depending on the combination of voltages applied to the contacts and the gate, the team could modulate the ratio of current flowing through the device when it was on or off by up to a factor of 10. (Silicon logic gates typically have on-off ratios that are 10 000 or so.)

But the proton transistor is different from conventional electronic transistors in a few key ways. For one, there are no p-n junctions to block current when the device is off. Instead, the device functions more like a wire with variable conductivity than as a perfect switch.

The device’s protons also move differently from ordinary electric current. The team suspects that instead of flowing freely across a transistor channel, the protons hop from molecule to molecule along ordered chains of water, helped along by hydrogen bonds that tie the protons to the next water molecule in the chain.

According to Rolandi, palladium was the key to getting the transistor to work, because it is one of a rare group of materials that can absorb hydrogen, creating a hydride that easily accepts and donates protons. Using the material to build the source and the drain allowed the team to inject protons into the channel and let them exit the device, just as in an electronic transistor.

Previous solid-state transistors, including a device presented in 2009 (pdf) by Michael Reznikov and Alex Kolessov of Physical Optics Corp., based in Torrance, Calif., have been used to control the flow of ions through a film. But the ions in such devices could only be made to go back and forth with an alternating current. The charges couldn’t enter or exit the device, says Ryan O’Hayre, a materials scientist at the Colorado School of Mines, in Golden. O’Hayre says his team has been working on a similar alternating-current device, but he notes that such transistors are more useful as sensors than switches and come with a range of physical limitations. "You run into all sorts of issues like parasitic capacitance," O’Hayre says.

This new transistor is a faster and more practical way to move protons than using microfluidics, says Aleksandr Noy of the University of California, Merced. "It’s relatively easy to fabricate, and it’s stable," Noy says. "It’s a big step."